readme: more tests (#1184)

* readme: more tests

Add dnssec and file plugin to the test readme. This requires creating a
bunch of files with the right content. Doing so already unconvered an
unconditional type assertion in DNSSEC. This PR will include the fix for
that as well.

Also extended the snippets in the file plugin README, so that they are
whole Corefile - showing more value and checking all corefile snippets.

Create outliner right now is the kubernetes plugin, because even setting
the right env vars will result in:

open /var/run/secrets/kubernetes.io/serviceaccount/token: no such file or directory":

Which we can't create for a test.

* lint

plugin/file/README.md

@@ -44,9 +44,22 @@ file DBFILE [ZONES... ] {
 Load the `example.org` zone from `example.org.signed` and allow transfers to the internet, but send
 notifies to 10.240.1.1
 
-~~~
-file example.org.signed example.org {
-    transfer to *
-    transfer to 10.240.1.1
+~~~ corefile
+example.org {
+    file example.org.signed {
+        transfer to *
+        transfer to 10.240.1.1
+    }
+}
+~~~
+
+Or use a single zone file for multiple zones:
+
+~~~
+. {
+    file example.org.signed example.org example.net {
+        transfer to *
+        transfer to 10.240.1.1
+    }
 }
 ~~~